Bulk acoustic wave (BAW) devices use the piezoelectric effect to convert electrical energy into mechanical energy resulting from an applied radio frequency (RF) voltage. BAW devices generally operate at their mechanical resonant frequency which is defined as that frequency for which the half wavelength of sound waves propagating in the device is equal to the total piezoelectric layer thickness for a given velocity of sound in the piezoelectric material. BAW resonators operating in the GHz range (e.g., at about 2 GHz) generally have physical dimensions of tens of microns in diameter with thicknesses of a few microns.
For functionality the piezoelectric layer of the BAW device is acoustically isolated from the substrate. There are two conventional device structures for acoustic isolation. The first is referred to as a Thin Film Bulk Acoustic Resonator (FBAR) device. In a FBAR device the acoustic isolation of the piezoelectric layer is achieved by removing the substrate or an appropriate sacrificial layer from beneath the electroded piezoelectric resonating component to provide an air gap cavity.
The second known device structure for providing acoustic isolation is referred to as a Solidly Mounted Resonator (SMR) device. In a SMR device the acoustic isolation is achieved by having the piezoelectric resonator on top of a highly efficient acoustic Bragg reflector that is on the substrate. The acoustic Bragg reflector includes a plurality of layers with alternating high and low acoustic impedance layers. The thickness of each of these layers is fixed to be one quarter wavelength of the resonant frequency. A variant of the SMR device adds a second Bragg mirror on the top of the piezoelectric resonator of BAW resonator. A conventional BAW oscillator leadframe package comprises a BAW die side-by-side with an oscillator circuit die that have bond pads which are coupled die-to-die by bond wires. Gold (Au) bond wires can be used for this die-to-die coupling.